I've found that "common sense" just means "things that I believe, but I can't explain why".

"Common sense is just the set of prejudices acquired by the age of eighteen."

~Albert Einstein

Yeah, this one annoys me no end. Especially as its used when workplace safety is concerned far too often.

A while back I was in an internet argument about a bicycle race in which a parked car caused a massive pileup. People were saying in the comments that it was entirely the cyclists' fault because they were all grouped up, and you never operate a vehicle if you can't see some arbitrary distance in front of you, and the car was parked! Common sense applies in common situations. In a long distance bike race, there's an assumption that the road is clear. It's common in these races to be shoulder to shoulder with absolutely minimal forward visibility.

A similar argument in that Alec Baldwin thing. "The four rules of firearm safety! Don't point it at anything you don't want to kill! Keep your finger off the trigger!" This was a movie set. It's common on movie sets for the firearms to be checked and rechecked and checked again before they make it on set. If you're at someone else's house and they hand you a gun to look at, common sense applies–make sure there isn't a magazine in, make sure there's nothing in the chamber, and still don't point it at your buddies. It's different on a movie set. The common assumption is that the armorer has checked all the guns on set, and that the crew haven't brought a bunch of live ammo to play with. Of course Baldwin should have checked the gun. And of course the cyclists shouldn't have been so close together. But in a million other movies on a million other sets, and a million other races on a million other tracks, this was never a problem.

I find the best retort to be: "Common sense ain't all that common."